How many saltwater fish can you have in a tank?

If you've gone to the trouble of buying all the right equipment and setting up your very own saltwater aquarium, you're probably keen to adopt lots and lots of fish. Most marine aquatics enthusiasts want their tanks to be as lively and as colourful as possible!

However, it's important not too put too many fish in one tank. An overcrowded aquarium is likely to have a lot of problems, including:

• Dangerously high ammonia and nitrate levels
• Insufficient oxygen
• Stunted fish growth
• High risk of stress and disease
• Fish behaving aggressively towards one another

So whatever you do, you need to make sure you don't overstock your tank. But that begs the question...

How Many Fish Is Too Many Fish?

Obviously, the answer to that question varies a lot depending on the size of your tank and what type(s) of fish you plan to put in it.

Different species grow to different sizes and have different requirements, so as ever, you will need to do your research before jumping in. Decide which fish you wish to keep in your marine aquarium, and read up on each species before purchasing - in particular, try to find out:

• How big each fish will be once fully grown
• How much space each fish needs
• How quickly you can add new species to your tank, and in what order

If you can get the answers to these questions, this should help you to calculate the maximum number of fish you can safely keep in your saltwater tank.

Rule of Thumb: How Many Fish Per Litre?

We can't tell you exactly how many fish to put in your tank, but if you're looking for a ballpark estimate, you will generally need to allow at least 1 litre of water per 1cm of fully-grown fish.

For example, if your tank's capacity is 100 litres, you probably shouldn't put more than 100cm of fish in there (e.g. you can have up to ten 10cm long fish, or up to five 20cm long fish).

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Coral-rich reef tanks with a sparing amount of fish have been the standard, but this author has found a way to successfully stock both coral and fish—including several tangs!

Filling Up the Tank

I have often heard reef aquarists joke about the small amount of fish that their aquariums can house. In the past it has been noted that in reef aquariums, invertebrates and corals are the stars and the fish take on a supporting role. In many such cases people are referring to the time when undergravel filtration, live rock, and an inefficient protein skimmer were the norms for water quality control.

While it’s still true that fish add waste—not just nitrogen compounds but also carbon dioxide—to any reef aquarium, today’s new technology and methodologies have changed the way reef aquariums are operated. Although the old way of thinking is still somewhat correct, it is possible to have a healthy reef aquarium well-stocked with both fish and corals.

This is especially important to me because I am a scuba diver who sees tons of fish on reefs throughout the world and thus considers them the heart and soul of the marine aquarium hobby; I could not imagine a tank favoring coral over fish life. With some attention to detail and an adventurous approach, you can have the best of both worlds—a reef aquarium well stocked with fish, corals, and other invertebrates.

The Natural Reef

Anyone who has visited a coral reef or has simply seen footage of one would quickly notice that these are not barren ecosystems. Being on a reef is somewhat like being in a big city with beltways, buildings, and a bustling population.

Biodiversity

Coral reefs are extremely diverse ecosystems, so diverse in fact that in coral reef biology the health of a coral reef is often measured by the amount of biodiversity it is home to. I have always sought to recreate this type of biodiversity in my aquariums. From a natural standpoint, many coral reefs appear overcrowded and dense, whereas many reef aquariums are crowded with coral but have few fish.

It is a given that the extent of biodiversity seen on a coral reef cannot be recreated in the home aquarium. The fact that the aquarium is a closed system without a constant ebb and flow to purify it alone prevents this. It isn’t impossible to create a very diverse captive ecosystem with a strong fish population, however.

Filtration

When looking at the various forms of filtration used in reef aquariums today, it is possible to devise a plan that will not only help export the waste created by fish but also add many of the elements that corals need to thrive. I have found that keeping a densely stocked reef resulted in fish that had more vibrant colors and personalities than when I kept my reef nearly devoid of fish life.

Walking the Tightrope

Make no mistake about it, keeping a densely stocked reef aquarium isn’t for everyone, and I wouldn’t recommend it for someone new to the hobby. I often consider keeping a densely stocked aquarium analogous to walking a tightrope. Going too far to the left or right spells disaster; you have to keep everything even and down the center.

Water Quality

Testing your nitrate, calcium, alkalinity, and pH values weekly becomes even more important, since a change in any of these principals can cause problems. And for densely stocked reefs, filtration must be oversized—selecting items rated for a tank twice the size of yours is essential, although it becomes expensive.

Stocking the Reef

So just how many fish can be kept in a densely stocked reef? The answer obviously depends on the size of your tank and the type of equipment you have. I personally have had good success keeping eight to nine fairly large-sized fish in a 120-gallon aquarium with a 50-gallon sump.

I know that many reef aquarists will probably gasp at this, but I have operated this system for well over nine years. With a regimen of testing and water changes, I have found it to be more than maintainable. In addition to that, I have several fish in my reef aquarium that are not commonly seen—among them a large blue-face angelfish Pomacanthus xanthometopon and a moderately sized polleni grouper Cephalopholis polleni.

Relationships Between Corals and Fish

The sad truth is that some fish eat certain corals. Of course all fish are individuals, but in most cases certain species will dine on certain corals, and adding a fish in with a coral it will eat results in failure. The good news is that not all fish eat corals, and even those that do often don’t eat every species of coral.

Another sad truth is that some fish eat other fish—the old rule of thumb is that if a fish can physically eat another fish, regardless of the species, it will. An old friend of mine who owns a fish outlet in Maryland always commented that “If a guppy was large enough to swallow a shark, it would.” Rule number one of keeping a peaceful tank is that you need to keep fish that are all similar in size and will, when they become adults, also be similarly sized.

The second rule of thumb in stocking fish is to keep fish species that are compatible. No matter how much filtration you have or how many water changes you perform, if you throw two powder blue tangs Acanthurus leucosternon into an aquarium together, they are going to fight and likely one won’t survive. Some trial and error is necessary to get the scale to balance, but normally you can depend on the experiences of others as a guide.

Evading the Tang Police

Many of you will be familiar with the “tang police”; there are normally many of them on every aquarium-related Internet forum. Some claim that no tang should be kept in an aquarium under 200 gallons, and others just feel that surgeonfish shouldn’t be kept at all. Naturally, when I say that I have not one, not two, not three, but four tangs in a 120-gallon aquarium, it attracts attention from the tang police.

Make Enough Space

First, I would never advocate keeping a fish you cannot house into adulthood. Many species of surgeonfish can live for a very long time if given proper care, and most species get large—too large even for an aquarium like the one I own.

Don’t Mix Surgeonfish of Different Groups

The trickiest part about keeping multiple surgeonfish is having the animals get along and not literally kill each other. The general rule is to not mix surgeonfish from the same genus together, such as multiple Acanthurus or Zebrasoma species. While this works as a general rule, you will often find that even surgeonfish from different groups will fight. I was able to achieve success by following that rule, however.

War and Peace

My first surgeonfish was a yellow tang Zebrasoma flavescens. Roughly a year later, I introduced an orange-shoulder tang Acanthurus olivaceus that was given to me by a local aquarist. The two got along beautifully. I then introduced a chevron tang Ctenochaetus hawaiiensis I purchased at a local aquarium store. War broke out among all three tangs. I removed both the yellow and orange shoulder tang and placed them in the sump (don’t worry, it’s a 50-gallon sump). After several weeks I placed them back in the display tank. There was peace throughout the reef.

Next I purchased a Pacific blue tang Paracanthurus hepatus. Again a declaration of war was made among my aquarium’s surgeonfish population. This time all the surgeons except for the Pacific blue tang were removed and placed in the sump, with the same result as before. It appeared that this peacekeeping effort was very effective. I haven’t introduced any more surgeonfish and have no intentions of doing so, but I have found that the population I currently have has remained peaceful.

Managing Water Quality

I am sure most people are curious about managing water quality in a reef tank with such a large fish population. The reality is actually much easier than most would imagine, as I have found that letting nature do the work is effective. Mother Nature is always solving more than one problem at a time and often uses the byproduct created during that solution to solve another problem.

Using a Sump

When we consider that entire septic and graywater systems have been implemented to purify human waste from water, we should easily be able to filter our reef aquariums. The first thing you need is a large sump. We all know that sumps add water volume to the aquarium, but they also offer the flexibility to implement various filtration alternatives.

As I said, my sump is 50 gallons in capacity, and it has three chambers. A refugium is another prerequisite to keeping such a large fish population. Not only is it an area for microfauna and tiny invertebrates to flourish, it also allows macroalgae to grow uninhibited. I have a very large refugium that is lit 24 hours a day.

Chemicals

Chemical media is also very important for this type of aquarium to serve as backups in case something goes wrong. Phosphate reducers, nitrate reducers, and high-end carbon are all important to keeping water quality in tip-top shape.

Protein Skimming

Protein skimming is one of your main allies in this scenario. Fellow aquarists are often shocked when they learn that I have three skimmers that run continually on my aquarium system. Not only are there a large number of skimmers on the system, but they are all sized for a much larger tank. While these units remove a lot of waste, they also remove a lot of the nutrients and elements that corals require to grow.

Water Changes

That is where we come to the most important part of managing a densely stocked aquarium: water changes! This is probably the single most expensive part about keeping a dense aquarium—as we all know, salt mixes are far from cheap. When a large fish population is creating problems with nutrients and your filtration is removing crucial elements that corals require, replenishing the water is really the only way to maintain water quality. In order to maintain acceptable water chemistry I perform three 10-percent water changes per week. Using only pure (RO/DI) water helps create a perfect saltwater mixture.

As you can imagine, changing 36 gallons of salt water weekly is tedious. I have tried doing water changes once per month or once every two weeks, but was simply unable to keep corals healthy when doing so.

Give and Take

Like many of life’s challenges, keeping a reef aquarium is sometimes a give-and-take proposal. If you are looking for a reef densely populated with coral life that only requires a monthly water change, the idea of keeping a large fish population likely won’t work. If the idea of having to remove fish and possibly reconfigure some of your aquarium’s logistics sounds daunting, then, again, this type of setup is likely not for you.

But if you really want to create the diversity and explosion of life that is often seen on natural coral reefs, perhaps a large fish population is exactly what you may be looking for. It’s exciting and always presents a unique challenge for the seasoned aquarist.

As I mentioned earlier, large fish populations in reef aquariums are not advisable for new hobbyists. It takes time (often years or more) to learn the basic principles of reefkeeping. Large fish populations complicate nearly every facet of reef aquarium maintenance, and it can be a daunting task simply to find a balance that works.

The Wise Words of Kurt Vonnegut

The great American novelist Kurt Vonnegut once said, “I want to stay as close to the edge as I can without going over. Out on the edge you see all kinds of things you can't see from the center.” In many ways keeping a reef aquarium containing a large fish population is standing on the edge. If you teeter too far forward, however, you will fall off.

With attention to detail, research, elbow grease, and a strong background as a reef aquarist, you can make these dynamic and exciting setups functional.

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